Types of Vaccine-derived Poliovirus
There are three types of vaccine-derived poliovirus [2,3]:
1. Circulating vaccine-derived poliovirus (cVDPV)
On very rare occasions, if a population is seriously under-immunized, and other risks factors are present (crowding, poor hygiene and tropical climate) the probability that the Sabin viruses can mutate or recombine and change to VDPV exists, if there are enough susceptible children for the excreted vaccine-derived polioviruses to begin circulating in the community. These viruses are called circulating vaccine-derived polioviruses (cVDPV).
As with naturally occurring poliovirus, the only protection against cVDPV is full vaccination. Circulating VDPVs in the past have been rapidly stopped with 2–3 rounds of high-quality immunization campaigns. The solution is the same for all polio outbreaks: immunize every child several times with the oral vaccine to stop polio transmission, regardless of the origin of the virus.
2. Immunodeficiency-related vaccine-derived poliovirus (iVDPV)
Prolonged replication of vaccine-derived viruses has been observed in a small number of people with rare immune deficiency disorders (ej: B cell immunodeciencies). Because they are not able to mount an immune response, these people are not able to clear the intestinal vaccine virus infection, which is usually cleared within six to eight weeks. They therefore excrete immunodeficiency-related vaccine-derived polioviruses (iVDPVs) for prolonged periods.
The occurrence of iVDPVs is very rare. Only 42 cases have been documented worldwide. Of these, most stopped excretion within six months or died. Three people excreted the virus for more than 5 years.
3. Ambiguous vaccine-derived poliovirus (aVDPV)
Ambiguous vaccine-derived polioviruses (aVDPVs) are vaccine-derived polioviruses that are either isolated from people with no known immunodeficiency, or isolated from sewage whose ultimate source is unknown. Very little is known about them but to go into this classification, there has to be no more AFP cases related to VDPV in the community.